In general terms, transmission systems operate to convert rotation, such as the rotation of an output shaft from an engine or other prime mover, into rotation at a different speed, or in a different direction, or both. Gearboxes are one common form of transmission system. One common use for gearboxes is to convert high-speed, low torque rotations into lower speed, higher torque rotations. Automobile gearboxes provide a good example of this.
Internal combustion engines used in conventional automobiles typically operate at engine speeds between 800 rpm and 7000 rpm. Hence, the speed of rotation delivered by the engine's output drive shaft (“crankshaft”) is between 800 rpm and 7000 rpm. However, for ordinary automobiles which travel at speeds between 0 km/hr and 120 km/hr, and assuming an outer diameter for the automobile wheels (including tyres) of approximately 40 cm, the automobile's wheels are only required to rotate at between 0 rpm and 1591 rpm. Furthermore, automotive internal combustion engines typically deliver maximum torque at engine speeds somewhere in the middle of the 800 rpm-7000 rpm operating range, whereas maximum torque is usually required to accelerate the car from stationary or low speed to a higher speed. Consequently, automobiles are typically provided with a transmission system or “gearbox” to convert the high-speed, low torque rotation delivered by the engine into a lower speed, higher torque rotation suitable for propelling the automobile.
Transmission systems are also used in a large variety of other machines and other mechanical applications which utilise rotation and where it is necessary to convert the rotation to a higher or lower speed, or to change the direction of rotation. Those skilled in this area will be familiar with other applications for transmission systems and therefore further applications need not be described. Also, it will be clearly understood that the invention is in no way limited to automobiles or any other particular application, and the automobile example above is given for the sole purpose of providing one illustration of an application of transmission systems.
Many transmission systems provide more than one conversion ratio between the speed of rotation delivered by the engine or prime mover and the resultant speed/direction of rotation after the rotation has been converted by the transmission. In gearbox type transmission systems, this is achieved by providing a series of gears of differing sizes, and the overall conversion ratio can be altered by causing differently sized gears to mesh with each other, thereby giving the transmission a different “gear ratio” depending on which combination of gears is engaged. The gears are typically contained within a casing, hence the common colloquial name “gearbox” for this kind of transmission system.
One of the major problems with transmission systems such as the gearboxes described in the previous paragraph is that they generally provide only a few discrete gear ratios. This is because each of the gears inside the gearbox is fixed in size, and therefore the number of possible gear ratios is limited to the number of different possible combinations of differently sized gears that can engage with each other. As an example, most automobile gearboxes have seven or fewer gear ratios (including the reverse gear). This can lead to problems or inefficiencies in applications where the ideal ratio between the speed of the rotational input to the transmission system and the speed of the rotational output from the transmission system does not correspond with one of the discrete ratios.
It is useful to refer again to the automobile example as one possible illustration of the problem described in the previous paragraph. Situations commonly arise where the transmission ratio that would allow the automobile engine to operate at optimum fuel efficiency for a required automobile speed does not correspond with one of the possible ratios of the automobile's gearbox. Therefore, in order for the automobile to travel at that desired speed, the automobile gearbox must be placed in a gear that provides a non-ideal gearing ratio, and the automobile engine must be run at a speed higher or lower than the engine speed which would provide optimal fuel efficiency. Those skilled in this area will recognize other problems or inefficiencies in other applications which arise because of the discrete gearing ratios available with these kinds of gearboxes.
There would therefore appear to be an advantage in providing a transmission system which is not limited to discrete gearing ratios, or which at least provides a large number of gearing ratios (preferably well in excess of seven, and preferably close to each other), such that the transmission system can be placed in a condition where its input-output ratio is (or is close to) that required for a particular operating speed in a given application.
Transmission systems have been devised which are not limited to discrete gearing ratios. In general, these different transmission systems operate such that the ratio between the speed of rotation delivered by the engine or prime mover and the resultant speed/direction of rotation after the rotation has been converted by the transmission system can be varied continuously, often within a given range. In other words, they operate to provide continuous or infinite variability in the transmission system's input-output ratio, again, often within the transmission system's operating speed range.
Of the transmission systems mentioned above which provide continuous variability in the system's input-output ratio, many are based on a variable-diameter pulley or “Reeves Drive” configuration. Reeves Drive type systems have a pair of rotating pulleys (one drive/input pulley and one driven/output pulley) and a belt running between the pulleys. Each pulley has two separate sides which, when assembled together, form a V-shaped track extending around the circumference of the pulley. The separate sides of both respective pulleys can be moved closer together and further apart as described below. The belt runs around the V-shaped track in each pulley as the pulleys rotate. The belt also typically has a V-shaped cross-section so that the sides of the belt have a similar slope to that of the V-shaped tracks. This enables the sides of the belt to contact closely against the sides of the V-shaped track on each pulley, thereby minimising slip between the belt and the pulleys.
The variation in the transmission input-output ratio in these Reeves Drive type systems is achieved by moving the sides of one pulley closer together and moving the sides of the other pulley wider apart. Doing this has the effect of increasing the width of the V-shaped track on the first mentioned pulley and decreasing the width of the track on the other pulley. If the sides of the drive/input pulley are brought together and the sides of the driven/output pulley are moved apart, this forces the belt outwards on the drive/input pulley and makes the belt move around that pulley at a greater diameter, the V-shaped track on the driven/output pulley widens allowing the belt to move around that pulley at a lesser diameter. This results in the drive/input pulley doing fewer rotations for every rotation of the driven/output pulley. Hence, this causes the transmission system to convert the input prime mover rotation into rotation at a higher speed. Conversely, if the sides of the driven/output pulley are brought together and the sides of the drive/input pulley are moved apart, this has the opposite effect (i.e. it causes the transmission system to convert the input prime mover rotation to rotation at a lower speed).
Hence, moving the sides of the pulleys as described above changes the effective diameters of pulleys, and therefore changes the input-output ratio of the transmission system. Those skilled in this area will appreciate that, in these Reeves Drive type systems, the distance between the two pulleys does not change, and neither does the length of the belt, so changing the input-output ratio means both pulleys must be adjusted (i.e. the effective diameter of one must decrease if the effective diameter of the other increases) simultaneously to maintain the proper amount of tension in the belt. One of the important aspects of these systems is that, because the spacing between the sides of each pulley is not limited discrete spacings, it is possible to create continuous variation in the input-output ratio of the transmission system by varying the space between the sides of the pulleys.
It is an object of the present invention to provide an alternative form of transmission system which allows continuous variability in the system's input-output ratio, or which can at least provide a large number of gearing ratios. The transmission system of the present invention may be adapted for use in a range of applications. However, it will be clearly appreciated that any reference herein to prior or existing transmission systems or any other background material or information does not constitute an acknowledgement or admission that any transmission systems or other information of any kind, or any combination thereof, ever formed part of the common general knowledge in the field, or is otherwise admissible prior art, whether in Australia or in any other country.